A 16-year-old Holsteiner gelding in Switzerland affected by neurological problems was diagnosed with tick-borne encephalitis. He made a good recovery, researchers report.
The case report, published in the journal Viruses, is the first to describe the diagnostic criteria in a horse as recommended in humans with suspected Tick-Borne Encephalitis Virus (TBEV) infection.
Tick-borne encephalitis is capable of causing neurological signs in humans and animals. Studies on its occurrence in horses include reports of non-specific and neurological signs. However, postmortem confirmation of infection in a horse with obvious neurological signs has been performed in only one early published case study.
Nathalie Fouché and her fellow researchers noted that the seroprevalence of the virus in the equine population in Switzerland has not been investigated to date. In other European countries, its seroprevalence in equines — indicating previous exposure to the virus — ranged from 2.9 to 37.5%.
The horse in the case study was referred for evaluation after showing a lack of co-ordination and lethargy. He also showed muscle tremors and was mildly sweating.
The gelding was treated with metamizole, which is a painkiller and spasm reliever, and the anti-inflammatory drug dexamethasone before his referral to the equine hospital.
At home, the horse had been kept in a stable with daily access to pasture with other horses in a rural area. The gelding never travelled abroad nor within Switzerland.
According to the owner, the horse was regularly affected by tick bites, except during winter.
At hospital, the horse was found to have an increased heart rate. During the first clinical examination, the animal was bright and responsive with normal mental activity. There were no cranial nerve deficits apart from a questionable mild facial nerve paralysis with drooping of the right upper and lower lip.
An asymmetry of the temporal muscle with a slightly decreased tone on the right side was also noted. The horse showed fine muscle twitches and tremors on the nostrils, lips, and eyelids.
The gelding was eating, drinking, defecating, and urinating normally.
In the first 24 hours of hospitalisation, the horse displayed periods of severe lethargy, during which incoordination with a drift towards the right side was observed when the horse walked. It was more pronounced in the hind than the forelimbs.
A complete blood count and serum biochemistry profile was undertaken, and a cerebrospinal fluid sample was obtained for analysis. Laboratory findings in the horse were indicative of viral infection and general inflammation.
Molecular-based testing of samples from the horse confirmed infection with TBEV, with evidence of acute and recent infection.
Treatment consisted of the nonsteroidal anti-inflammatory drug flunixin-meglumine, the steroid prednisolone, and vitamin E.
The horse remained hospitalized for six days and continued to show muscle twitches and periods of lethargy, during which a lack of co-ordination was still observed.
The clinical signs gradually became less pronounced and less frequent and were absent by the end of the hospital stay on day six. The horse was discharged, but continued on the medications for another two weeks.
Three weeks after discharge, the horse was presented for a re-examination. Vital parameters were within normal limits and the horse did not show any signs of lethargy, according to the owner.
The neurological examination was normal except for a slight persisting facial asymmetry with drooping of the nostril on the right side.
The owner reported that the gelding showed no signs of disease 12 months after discharge.
“In conclusion, this is the first report to describe ante-mortem diagnosis of TBEV in a horse based on the diagnostic criteria recommended in human medicine.
“This diagnostic protocol may be implemented to investigate TBEV infection in horses living in endemic areas and showing compatible neurological signs.
“We recommend implementing this procedure in the diagnostic work-up of neurological cases with an uncertain etiology (cause) in combination with seasonal tick activity after ruling out a potential infection with West Nile Virus, even in countries that are yet considered free of West Nile Virus infections.”
Discussing their findings, the authors noted that recent serological and PCR-based surveys in horses suggest that only a few horses exposed to, and infected with, the virus develop neurological signs — which is similar to humans.
Nevertheless, increasing awareness and testing for West Nile virus infection (and associated testing for TBEV infection) in Europe has also raised suspicion for TBEV-related neurological disease.
“However, to date, reports about ante-mortem diagnostic investigations of suspected cases are limited. Increasing awareness of the disease should encourage equine veterinarians to investigate suspected clinical cases according to the human diagnostic guidelines.”
Cases of the virus are defined by the presence of clinical signs of meningitis, meningoencephalitis or meningoencephalomyelitis, with an increased white blood cell count in the cerebrospinal fluid, the presence of specific antibodies against the virus, including in the cerebrospinal fluid, or TBEV Immunoglobulin G seroconversion.
Treatment in humans and animals focuses on easing the signs of disease, as there is no antiviral treatment available.
The horse in the case study responded well to a combination of anti-inflammatory drugs and anti-oxidative therapy, the researchers said.
Disease prevention is currently limited to reducing the contact to potentially infected ticks in endemic areas.
The case report team comprised Fouché, Solange Oesch and Vinzenz Gerber, with the Swiss Institute of Equine Medicine at the University of Bern; and Ute Ziegler, with the Institute of Novel and Emerging Infectious Diseases, which is part of the Friedrich Loeffler Institute.
Fouché, N.; Oesch, S.; Ziegler, U.; Gerber, V. Clinical Presentation and Laboratory Diagnostic Work-Up of a Horse with Tick-Borne Encephalitis in Switzerland. Viruses 2021, 13, 1474. https://doi.org/10.3390/v13081474